Leuco base

Leucoanthocyanins Leuco azomethines Leuco bases Leucocytes... 

There are many colorimetric methods used for trace analysis of peroxides using reagents such as ferrous ion, leuco base of methylene blue, yy -diphenylcarbohydrazide, titanium(IV), iodide ion, and Ai,A7-dimethyl- -phenylenediamine. The latter two are the most commonly used reagents... 

Reduction. Triaryknethane dyes are reduced readily to leuco bases with a variety of reagents, including sodium hydrosulfite, 2inc and acid (hydrochloric, acetic), 2inc dust and ammonia, and titanous chloride in concentrated hydrochloric acid. Reduction with titanium trichloride (Knecht method) is used for rapidly assaying triaryknethane dyes. The TiCl titration is carried out to a colorless end point which is usually very sharp (see Titanium COMPOUNDS, inorganic). 

Cl Acid Green 5, is still made by trisulfonation of the leuco base using ethylben2yl aniline and ben2aldehyde as starting materials. 

Spkit Blue [2152-64-9] Cl Solvent Blue 23 (Cl 42760), is one of the few dyes sulfonated as the leuco base. The degree of sulfonation depends on the conditions. Monosulfonated derivatives, commonly referred to as alkaU blues, eg. Cl Acid Blue 119 [1324-76-17, are used as thek barium or calcium salts in printing inks. Disulfonated compounds, eg. Cl Acid Blue 48 [1324-77-2] are employed as thek sodium or ammonium salts for blueing paper, whereas the trisulfonic derivatives or ink blues, eg. Cl Acid Blue 93 [28983-56A] are used in writing inks (qv). 

The preparation of triaryknethane dyes proceeds through several stages formation of the colorless leuco base in acid media, conversion to the colorless carbinol base by using an oxidising agent, eg, lead dioxide, manganese dioxide, or alkah dichromates, and formation of the dye by treatment with acid (Fig. 1). The oxidation of the leuco base can also be accompHshed with atmospheric oxygen in the presence of catalysts. 

Fig. 1. Preparation of triaryknethane dyes through the colorless leuco base.

The central carbon atom is derived from an aromatic aldehyde or a substance capable of generating an aldehyde during the course of the condensation. Malachite green is prepared by heating benzaldehyde under reflux with a slight excess of dimethyl aniline in aqueous acid (Fig. 2). The reaction mass is made alkaline and the excess dimethylaniline is removed by steam distillation. The resulting leuco base is oxidized with freshly prepared lead dioxide to the carbinol base, and the lead is removed by precipitation as the sulfate. Subsequent treatment of the carbinol base with acid produces the dye, which can be isolated as the chloride, the oxalate [2437-29-8] or the zinc chloride double salt [79118-82-4]. 

The starting materials of the aldehyde method may be sulfonated. For example. Cl Acid Blue 9 [2650-18-2] Cl Food Blue 2 (Cl 42090), is manufactured by condensing a-(A/-ethylanilino)-y -toluenesulfonic acid with o-sulfobenzaldehyde. The leuco base is oxidized with sodium dichromate to the dye, which is usually isolated as the ammonium salt. In this case, the removal of the excess amine is not necessary. However, this color caimot be used in the food sector because separation of the chromium compounds from the dye is difficult. An alternative method which gives food-grade Cl Acid Blue 9 (14) and dispenses with the use of sodium dichromate employs oxidative electrolysis of the leuco base (49). 

Diphenylmethane Base Method. In this method, the central carbon atom is derived from formaldehyde, which condenses with two moles of an arylamine to give a substituted diphenylmethane derivative. The methane base is oxidized with lead dioxide or manganese dioxide to the benzhydrol derivative. The reactive hydrols condense fairly easily with arylamines, sulfonated arylamines, and sulfonated naphthalenes. The resulting leuco base is oxidized in the presence of acid (Fig. 4). 

V-methyl aniline formed in the initial step to give the leuco base of methyl violet. Treatment with aqueous acid produces the dye. Because Michler s hydrol may also react with dimethyl aniline instead of the /V-methyl aniline to give crystal violet, commercial-grade methyl violet is usually a mixture. A cobalt complex has converted 4,T-dimethylaminodipheny1methane and dimethyl aniline in the presence of atmospheric oxygen to crystal violet in one step (50). 

A similar sequence starting with the leuco base of tetrahydroxyanthraquinone affords the very potent... 

The almost colourless leuco form of the base passes upon oxidation into the strongly coloured indamine. When titrating iron(III) at a pH of about 3 and the colourless hydrochloride of the leuco base is added, oxidation to the violet-blue indamine occurs with the formation of an equivalent amount of iron(II). At the end point of the EDTA titration, the small amount of iron(II) formed when the indicator was introduced is also transformed into the Fe(III)-EDTA complex FeY-, whereupon the blue indamine is reduced back to the leuco base. 

The Ce(IV) oxidation of the leuco-base of Malachite Green yields a dye, viz. 

The kinetics for the oxidation of leuco bases using oxygen has been studied.19 The oxidation involves complex formation between the proto-nated leuco base and the peroxy radical formed by air oxidation of the solvent. Addition of a radical initiator (AIBN) facilitates the reaction, while radical inhibitors retard the dye formation. In addition, oxidation reactions employing 2,3-dichloro-5,6-dicyanoquinone have shown large isotope effects in acetonitrile.20... 

The formation of colored materials from leuco bases such as 4 and 5 is accomplished by treatment with acids such as acid clay, bisphenol A, acetic acid, or silica gel.39 For leuco base 4 the leaving group is hydroxy, alkoxy, or cyanide, or a nitrogen-containing heterocycle. 

Hydroxy leuco bases can be converted into the corresponding amino leuco bases by allowing the leuco compound to react with a secondary amine in the presence of acetic acid.78 Examples, 54, of amine bases utilized in this manner are imidazole, 1,2,4-triazine, aryl amine, and cyclohexayl amine. 

Sulfonated triphenylmethane leuco dye 73 is prepared42 by sulfonating the leuco dye 74 with 20-30%of the theoretical amount of 20 - 65% oleum at 30 - 50°C. The dye, derived from leuco base 73 by oxidation, is used in the... 

Smith, I. L., Analytical Applications of the Heavy Metal Induced Oxidation of the Leuco Bases of Triphenylmethane Dyes, Ph.D. dissertation, The University of Alabama (1974) Chem. Abs. 83(8), 71097m. 

The two limiting structural forms of polyaniline are shown in Figure 12B (i) the nonconjugated leuco base polymer in which the... 

Condensation of a diphenylmethanol derivative, such as Michler s hydrol (6.159), with a reactive aryl component under acid conditions (the hydrol synthesis) also provides a leuco base. The dye 6.163 (Cl Acid Green 50) is made by reacting Michler s hydrol with R acid (2-naphthol-3,6-disulphonic acid) and oxidising the resulting leuco compound (Scheme 6.30). 

The nucleophilic displacement of suitable para substituents in analogues of malachite green is used as a means of synthesis of some important acid dyes. For example, Cl Acid Blue 83 (6.176 R = H) is made by the aldehyde method, using 4-chlorobenzaldehyde and N-ethyl-N-(3-sulphobenzyl)aniline. The resulting leuco base is oxidised to the colour salt,... 

Technically important dyes are salicylic acid derivatives that function as chrome mordant dyes for wool. Thus Cl Mordant Blue 1 (6.187) is made by the aldehyde synthesis from 2,6-dichlorobenzaldehyde and 2-hydroxy-3-methylbenzoic (o-cresotinic) acid in concentrated sulphuric acid. Oxidation of the leuco base is achieved by the addition of sodium nitrite. On wool the product, which is isolated as the sodium salt, is a dull maroon colour, changing to a bright blue on treatment with a chromium salt. Some dyes of this type, such as Cl Mordant Violet 1 (6.188), also contain a basic group. This compound is also prepared by the aldehyde route. 

Reduction to form a leuco base, removal of impurities, and subsequent reoxidation. 

Another route proceeds via bis(dimethylaminophenyl)methane, which is synthesized from dimethylaniline and formaldehyde. The thus prepared intermediate may be oxidized to form the hydrol and then reacted again with dimethylaniline to afford the leuco base, which is finally oxidized to yield 127 ... 

The aqueous solution of methylene blue is not decolorised by sodium hydroxide the blue water-solubie ammonium base is formed. The dye is converted by reducing agents into the easily oxidisable leuco-base. The following experiment, which demonstrates the formation of methylene blue by the introduction of sulphur between the rings of Bind-schedler s green, is very instructive. 

Preparation of the Leuco-Base—Zinc chloride (10 g.) is fused in a porcelain basin, cooled, and powdered. It is then added to a mixture of 25 g. of dimethylaniline and 10 g. of benzaldehyde (both freshly distilled) and the whole is heated in a porcelain basin on the water bath with frequent stirring for four hours. By the addition of hot water the viscous mass is liquefied on the water bath and the hot liquid is poured into a half-litre flask into which steam is passed until drops of oil cease to distil. After the liquid has cooled the water is poured off and the residue is washed several times with water. When as much as possible of the water has been removed the material in the flask is dissolved by adding alcohol and warming on the water bath, and the solution is filtered. On leaving the filtrate over night in a cool place the base separates in colourless crystals which are collected at the pump, washed with alcohol and dried in air on several folds of filter paper. A second crop of crystals can be obtained by concentrating the mother liquor. Should the base not crystallise, but separate as an oil—as often happens after the filtered solution has stood for a short time—it follows that too little alcohol has been used. In such cases somewhat more alcohol is added and the mixture is heated until the oil dissolves. Yield 20-24 g. 

Oxidation of the Leuco-Base.—In a round-bottomed flask (capacity 1-5 1.) 11 g. of the dried preparation are dissolved in 80 c.c. of hot 2 N-hydrochloric acid and the practically colourless solution is diluted with 800 c.c. of water. To the diluted solution, which is kept cool in ice and continually shaken, a suspension of 9 g. of lead dioxide in 30 c.c. of water is added slowly in small portions. Since commercial lead dioxide often reacts badly the oxidising agent should be prepared in the laboratory by the method given below. 

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